Minimizing the power consumption in mobile communication stations is important at the technical level because it is the battery life which often limits the available operation time of communication stations. Therefore, e.g. in mobile radio telephones, the stand-by state is commonly used, whereby the transmitter is cut off while waiting for a call. On the other hand, a battery saving system is known where, according to the quality of the connection, the telephone can be piloted by the base station to select its transmission power in two or more steps, so that the lowest possible power to produce satisfactory quality will be used. The power steps could, for instance, be 0,1 W; 1,0 W; 0,15 W; 1,5 W or 15 W. The use of a lower transmission power will contribute to the lowering of the power consumption and lengthen the available call time of the radio telephone accordingly.
For example, it is known to use a Mobile Assisted Handoff (MAHO) function in dual mode (analog and digital) cellular mobile telephone systems. More particularly, the EIA/TIA Interim Standard entitled "Cellular System Dual-Mode Mobile Station-Base Station Compatibility Standard" IS-54-B (April 1992) specifies the MAHO function in Section 2.4.5. Briefly, the MAHO function requires a mobile station to furnish RF channel signal-quality information to a fixed site base station that serves the mobile station. There are two types of channels upon which the mobile station performs signal measurements: (a) a currently assigned forward digital traffic channel, and (b) any other specified forward RF channel. The digital traffic channel has a Time Division Multiple Access (TDMA) format having six time slots that occur within a 40 millisecond frame.
For instance, where several base stations have overlapping coverage areas, each base station transmits with a unique, predetermined channel frequency, the channel frequencies being spaced apart from one another by 30 kHz. When the MAHO function is activated, the mobile station measures the received signal strength (RSSI) and estimates the Bit Error Rate (BER) of the current forward digital traffic channel. The MS also measures the RSSI and estimates the BER for the other channel during idle time slots. The various measurements and estimates made by the MS are employed to determine a next base station to which the MS will be assigned during a handoff procedure.
Today, the mobile communication stations are used in cars and other vehicles but also, their sizes having become smaller, in an increasing extent as portable pocket stations. Their use has increased in large office buildings, factories, railway stations and airports, and so on, when the interferences of the operational environment may require an increase in transmission power which will, in a disadvantageous way, shorten the available operation time of the station between battery chargings. On the other hand, there are also cases where radio telephones are desired to be continuously kept in the call state, and then the power consumption will be high.